Originally Posted by Gebo
Does it take a special soap?
What makes one better than another?
Yes, but also no. Certain brands do make specific "foam cannon" soaps (e.g. Chemical Guys' Honey Dew Snow Foam) that focus on creating thicker suds, but plenty of your off the shelf shampoos will work just fine. For example, I've been using Meguiar's Gold Class Shampoo in my foam cannon for years now with great results, and they only recently starting printing "great for snow foam cannons" on the bottles.
To put it simply, typically what people look for in an effective snow foam is thick and frothy suds, which result in a longer dwell time on your car's paint, allowing for the soap to carry away as many contaminants as possible.